┌─────────────────────────────────────────────────────────────────────┐ │ PLATE 12. │ │ │ │ [Illustration: _A._ V-shaped cañon cut in an upland recently │ │ elevated from the sea, San Clemente Island, California (after W. S. │ │ Tangier-Smith).] │ │ │ │ [Illustration: _B._ A “hogback” at the base of the Bighorn │ │ Mountains, Wyoming (after Darton).] │ └─────────────────────────────────────────────────────────────────────┘
FIG. 288.—Map of Santa Catalina Island, California, showing the characteristic surface of an area which has long been above the waves, and the entire absence of coast terraces (after U. S. C. and G. S.).]
=The Blue Grotto of Capri.=—We may now return to the Bay of Naples for additional evidence that oscillations of level in neighboring portions of the same coast are not necessarily synchronous, and that near-lying sections may even move in opposite directions at the same time, as has already been shown for the islands off the California coast. For the Pozzuoli shore of the bay it was learned that within the Christian Era a complete cycle of downward, followed by later upward, movement has been largely accomplished. Across the bay, and less than 20 miles distant, is the Blue Grotto of Capri, a sea cave cut in limestone above an earlier cave of the same nature which is now deep below the water surface. It is the refracted sunlight which enters the cave through this lower submerged opening and has been robbed on the way of all but its blue rays which gives to the famous grotto its special charm (Fig. 289).
FIG. 289.—Cross section of the Blue Grotto on the Island of Capri, showing the submerged sea cave through which most of the light enters the grotto, and the higher artificial window now widened by wave action (after von Knebel).]
It is known that the former, and now submerged, sea cave was in use by Roman patricians as a cool retreat from the oppressive hot wind known as the sirocco, and that an artificial entrance or window was cut where is now the only accessible entrance to the grotto. In the ancient writings, no mention is made, however, of the remarkable blue illumination for which it is now famous, and the conditions at the time, as we may see, were not such as to make this possible. Later subsidence of the coast has brought the ancient window to the sea level, where it has been considerably enlarged by the waves. The earlier grotto, abandoned as its entrance was closed, was rediscovered in 1826 by the painter and poet, August Kopisch.